Given the public policy goal is to prevent hospitals being overwhelmed due to admissions with the virus (and the commensurate decline into ICU on a ventilator etc.) the vaccine is doing two things - firstly it's reducing the number of people catching the virus. Secondly, it's reducing the number of people being hospitalised with Covid. That then has a follow-on impacat that there's a reduction in deaths (though that data isn't complete yet for the AZ vaccine, given timeframes, and the need to wait long enough before measuring mortality outcomes).

With good results from a single dose, even after a period of time, in a sense these people are "vaccinated". If the UK needs to re-vaccinate everyone with a booster twice once every 180 days (for example), that's "only" 378k vaccine doses per day, and they can deliver at well above that rate. If the supply increases as projected, there is potential for as many as 4m doses a week to be getting administered by April, with everyone having their first dose by June.

Pressure is then reduced on hospitals (as long as the vaccine is effective against prevalent strains) as you roll out subsequent doses - the 1st dose figures seem promising.

https://www.gov.uk/government/news/new-data-show-vaccines-re... for over 80s

https://papers.ssrn.com/sol3/papers.cfm?abstract_id=3790399 for healthcare workers under 65